When the 2012 lockout murdered half a season of hockey (and stole from us even more games from Pavel Datsyuk's career), one of the big changes that the NHL and NHLPA fought over was a way to set maximum contract term limits and also fix a loophole in the previous CBA called "backdiving." What had become a contentious point eventually got worked out into what colloquially became known as the "Luongo Rule," named for what was then probably the most-popular example of backdiving that the league actually allowed to happen without punishment.
What is Backdiving?
Simply put, when a contract tapers down from the first few years to a much lower annual salary, this is backdiving.